There are many applications for monitoring an optical fiber to detect physical disturbances, including monitoring for attempts to access data signals being transmitted over optical fiber, monitoring for attempts to intrude upon a zone protected by an optical fiber deployed around it, monitoring of vibration in machinery with which the optical fiber is associated, monitoring pipelines along which the fiber is distributed, and so on.
In the context of an optical fiber transmitting data, for example, it is well-known that an optical fiber can be tapped, and light extracted, without interrupting the transmission of the data signal along the fiber. One approach is to bend the fiber to such a degree that some light leaves it and can be diverted to a suitable receiver. Such a bend may introduce a loss of up to 1 dB of the transmitted signal, and optical data networks have intrusion detection systems capable of detecting this power loss and providing an intrusion alarm.
An alternative, lower-loss approach to tapping an optical fiber involves carefully grinding away the outer part of the fiber in order to permit evanescent-wave coupling to an adjacent tapping waveguide. The power loss resulting from this kind of tap might be as low as 0.04 to 0.004 dB. Such a low loss might be impossible to detect reliably by intrusion detection systems which use power measurement methods.
An alternative class of detection techniques is predicated upon the fact that physically disturbing an optical fiber will cause changes in the propagation characteristics of the optical fiber for polarized light. Typically, the physical disturbances will be asymmetric, so the refractive index for one state of polarization of the light will be altered differently from that for another state of polarization, causing relative phase shifts that will generally change the polarization state of a transmitted light beam that is composed of a mixture of these two polarizations. More particularly, in the various applications of the invention mentioned above, bending, compression, twisting or torqueing of the fiber may cause stress and strain that induce varying amounts of birefringence (linear or circular), resulting, for example, in a linear polarization state becoming elliptical or rotating. Such polarization changes can be detected as indications of a relevant disturbance of the optical fiber. Typically, sensors of this type are made using common telecommunications fiber, and using polarized light at a standard wavelength such as 1310 nm or 1550 nm for which inexpensive lasers are readily available, or at least between wavelengths such as 1260 nm and 1650 where the fiber operates in a single-mode manner and the attenuation is reasonably low.
Optical fibers carrying data usually are in a cable, sometimes with other fibers, and either of the above-described tapping procedures would entail physically disturbing the cable to such an extent that it is viable to design a monitoring system with enough sensitivity to detect an abnormal change in the polarization of light transmitted by the fiber to be tapped, or by a separate monitored fiber in the same cable, and initiate appropriate action, such as triggering an alarm, even before the intruder actually gained access to the fiber to be tapped.
Although, on the face of it, such a detection procedure might seem straightforward, in practice it is difficult to differentiate between normally-occurring disturbances and abnormal or “reportable” disturbances which should be reported because, for example, they result from a genuine intrusion attempt. The difficulty arises because effects in the fiber's environment can cause optical changes with similarities to some of those caused by intrusions. Such effects might include the effects of wind on a suspended cable, “stick-slip” phenomena combined with expansion and contraction during temperature changes, vibration from nearby equipment, or traffic or construction near a buried cable, and so on.
U.S. Pat. No. 7,173,690 (Haran) discloses a perimeter intrusion detection scheme comprising an optical fiber disposed around the perimeter of a building or other structure, for example by attaching it to a fence or burying it in the ground. Haran injects polarized light pulses into the optical fiber and detects changes in Rayleigh backscatter for different polarization states to determine occurrence of a disturbance and its location.
Haran uses a polarization optical time-domain reflectometer (POTDR) to capture a number (2 to 1000) of POTDR traces, digitally filters them and averages the digitally-filtered traces to form a “reference” trace which represents the normal or “no disturbance” condition. Thereafter, subsequent POTDR traces are acquired and compared with the reference trace. This process may take a typical POTDR several seconds (if not minutes) in order to achieve the desired signal to noise ratio (SNR) on a backscatter trace that is used for determining whether handling has occurred. As the length of the monitored fiber increases, a better SNR of the system is required, and therefore more averaging time. In view of the time taken to acquire enough samples for analysis, this method is mainly suitable for detecting slow changes, and is poorer for characterizing activity such as bumps or rattling that are associated with preparations to tap a fiber or other intrusions that have faster changes characterized by variations at moderate acoustic frequencies.
Other known techniques, such as those disclosed in U.S. Pat. Nos. 7,142,737, 7,206,469 and 7,693,359 and U.S. Patent Application Publication No. 2006/0153491, all by C. R. Murphy et al., and U.S. Pat. No. 5,061,847 (Hazan et al.) may sacrifice the position resolution of OTDR methods, but they require less signal-averaging time and can therefore capture acoustic-frequency disturbances with a good signal-to-noise ratio. This higher frequency response widens the range of criteria that can be used for distinguishing reportable disturbances from other changes. These techniques entail launching polarized light into an optical fiber, and detecting light emerging from the fiber through a polarizing filter or analyzer. The detection signal varies when the fiber is disturbed because a disturbance changes the transformation or rotation that the fiber enacts on the polarization of the light, so that, under the right conditions, the fraction of the light that passes through the analyzer will change.
The methods of Haran, of Murphy et al. and of Hazan et al. are subject to the phenomenon of “fading”: the sensitivity of the detection signal to a disturbance in the fiber depends strongly on (i) how the polarization state of the light entering the disturbed region happens to align with the geometry of the disturbance, and (ii) how the change in the polarization of the light emerging from the fiber happens to align with the polarization analyzer. (Alignment of polarization states with polarization transforming devices and filters is different from ordinary alignment in physical space, but is a well-known concept.) The alignment is generally unpredictable depending on how different parts of the deployed fiber happen to be bent, twisted or stressed; and it will vary as environmental factors change (such as the temperature of the optical fiber cable). Thus, the sensitivity to a particular disturbance will be unpredictable and will vary randomly over time, between substantially zero and some maximum value. This unpredictable and variable sensitivity can make it difficult or impossible to set a threshold that the observed magnitude of a disturbance must exceed to make it reportable; specifically that is low enough to ensure reporting of substantially all disturbances that should be reported, yet high enough to reject changes that should not cause reporting.
Murphy et al. use detectors with two different polarization analyzers at the output of the fiber, chosen so that, when a polarization change occurs in the light that reaches the output, if it is aligned so that one detector sees little or no change at all, the other detector will usually see a more substantial change. The resulting sensitivity may vary, but at least it will go near or all the way to zero considerably less frequently for polarization changes that reach the fiber's output. That is because it requires a coincidence where both detectors are insensitive to change. However, the polarization of the light passing through the disturbed region may still be aligned more often with a physical disturbance in such a way that little or no polarization change is created, so it is still possible for the sensitivity to go all the way to zero, and close to zero fairly often.
Of the three references cited above (Haran, Murphy et al. and Hazan et al.), only Hazan et al. address the problem of fading in a concerted way. Related work is published in their paper “Buried optical fiber pressure sensor for intrusion detection,” in Proceedings of the 1989 International Carnahan Conference on Security Technology, pp. 149 to 154, section 4.1.3. In this paper and their U.S. Pat. No. 5,061,847, Hazan et al. explain the causes of fading in detail, and disclose a way to mitigate the effects. Nevertheless, such mitigation is not entirely satisfactory.
Hazan et al. do better than Murphy et al. by using two launch states of polarization (as well as using two polarization filters for detection). Although Hazan does not state it this way, the two launch states should have Stokes vectors that are linearly independent. Thus, if the light due to one of the launch states, when it reaches the disturbed zone, is aligned with the disturbance in a way that produces no polarization change, the light from the other will necessarily be aligned in a way that does produce a change (and at least one of detectors at the output is likely to see a change). This use of two launch states means that a disturbance will always create a non-zero polarization change for at least one launch state, and thus makes a substantial improvement. Hazan et al. disclose several simple ways to combine multiple signals from different combinations of launch state and analyzer state (e.g. adding absolute values, or creating a logical “OR” of the alarms from two separate signals). However, although such ways of combining the signals (each of which has severe fading by itself) may reduce fading, significant fading can still occur, resulting in significant uncontrolled variations in sensitivity. (The aforementioned patent and paper by Hazan et al. do not clearly explore this limitation.) Consequently, this approach of Hazan et al. may not be entirely satisfactory, particularly in an application where it is not enough merely to reduce fading, but it is desirable to effectively eliminate it.
Hazan et al. and Murphy et al. specify rather explicitly what polarization states should be launched into the fiber, and what polarization filters or “analyzers” should be used at the detectors. Thus, Hazan et al. use a pair of linear polarization states at the input and another pair for polarization analysis at the output, with the second linear polarizer in each pair at 45 degrees to the first; or at least within the range of 30 to 60 degrees. In another case, they specify adding a circular polarizer. Murphy et al. teach the use of a pair of polarizers at detectors, one of which is linear and the second of which is either a circular polarizer or a linear polarizer at 90 degrees to the first. (In fact, it appears that this 90-degree angle does not increase the likelihood that every polarization change will cause a signal change, but merely provides redundant information: after correction for any sensitivity difference, the sum of the signals from two linear polarizers at 90 degrees is constant, proportional to the total power, so either one predicts the other.)
Such specific design constraints on the launch states and analyzer states can cause undesirable challenges in the engineering, specifying and manufacturing of a product. For example, some convenient ways to split beams to be divided between, and coupled to, different polarization analyzers, including coiled optical fibers used for coupling, may involve phase shifts that cause the combination of a linear polarizer with its coupling means to be tuned to some elliptical polarization state, and to have a relationship to a second polarizing filter that cannot be characterized so simply as a 45-degree angle between two linear polarizers. Similarly, analyzer means comprising a circular polarization filter or analyzer, when combined with its associated splitting or coupling means, may turn out to be tuned to a state that is very different from a circular polarization state. Almost all of the possible polarization states are elliptical to some degree, and tolerances on the departure from perfect linear or circular states would be a concern.
In summary, such previous polarization-based optical fiber disturbance monitoring systems experience significant fading, reducing their accuracy, consistency and reliability in assessing the magnitude of a disturbance, and hampering the ability to distinguish between reportable disturbances and disturbances that should not be reported. In alarm systems, this can either increase the frequency of false alarms or make it easier for an intruder to avoid detection. Such previous systems also place unnecessary restrictions on the polarization states launched into the fiber and/or on the filters or analyzers at the output end.
An object of the present invention is to at least mitigate deficiencies of such previous optical fiber disturbance monitoring methods and systems, or at least provide an alternative.